This invention relates to an apparatus for an out-of-round circumferential machining of workpieces, particularly piston rings. The apparatus includes a tool holder which is driven by a servomotor or a setting motor controlled numerically and electronically as a function of the desired out-of-round contour of the workpiece.
For the out-of-round machining of the inner and outer circumference of piston rings, machines have been known which in essence correspond to a lathe. The piston rings which in most instances can be arranged in axially tightened stacks are simultaneously machined at their outer and inner circumference, for example, as disclosed in U.S. Pat. No. 2,932,227. According to the disclosure therein, the tools are mounted at the ends of pivotal tool holders and the radial feed is controlled by a cam which rotates synchronously with the work spindle. For the scanning of the cam shape there is provided a follower roller which actuates a two-lever rocker. The follower roller transmits the control displacements from the cam to the rocker which carries the tool bit for the inner and outer circumferential machining of the piston ring stack. A spring urges the follower roller against the cam face in order to prevent the roller from lifting off the surface of the cam at higher cam rpm's. It is a disadvantage of such a purely mechanically operating out-of-round copying apparatus that--because of the relatively large masses to be accelerated by the cam as it rotates, particularly in the gap zone of the piston rings--a relatively low limit is placed on the maximum rpm of the work spindle. As a result, dependent upon the particular piston ring diameters, different cutting speeds are obtained.
The limited cutting outputs of machines operating with a mechanical copying of the tool feed are far below the possibilites offered by up-to-date tool materials, particularly ceramic cutting plates.
It is, to be sure, known to control the tool feed for the out-of-round circumferential machining numerically with the aid of data stored on punched tapes. For this purpose, according to German Laid-Open Application (Offenlegungsschrift) No. 2,006,760, at least two setting motors have been used whose setting motions which are individually numerically controllable, are superimposed by a gearing and are then transmitted to the cutting tool. The use of a plurality of setting motors and thus the required gearing not only represent a structurally very complex arrangement but further, relatively large masses have to be accelerated, so that such systems are limited to applications where only slight out-of-round variations occur, such as, for example, the circumferential machining of pistons for internal combustion engines. The acceleration peaks of the radial feed motion for the tool are therefore relatively small because of the quasi-oval out-of-round contour of the pistons.